The answer is fairly easy.
Hit CTRL-ALT-DEL to bring up your Task Manager. Enable "Hide when minimized" and choose "Low update.."
Now leave this running in the background when you play games. A game is usually CPU or Graphics Card limited; in a well balanced system some games will be CPU limited and others will be graphics limited.
Let's pretend you have a single core for a second. If you play a game and it never hits 100% then you are limited by your graphics card (unless you are not stressing either which is unlikely in most games).
Multiple CPU cores is slightly different as some games can use multiple cores and some can't.
1) No cores reach 100% means you are definitely Graphics limited.
2) One core reaches 100% and other cores are being used at least 20%. It's hard to say. You could be graphics or CPU limited.
3) Two of the four cores are at 100%. Probably CPU limited.
*In a particular game if you hover around 60% CPU usage then you could add a second card in SLI or get a faster one. Too fast and it's a waste of money because your CPU can't keep up so the Graphics card never gets used fully.
Hope this helps.
(You've got a nice system there. There's obviously much faster but any game would look nice with your system. Note, it's always best to turn down the graphics slightly rather than have lag. AA, Shadows, Bloom and Motion Blur are all prime candidates to disable or turn down.)